SIP 408 Request Timeout and Long Post-Dial Delay

What it means (general SIP)

408 Request Timeout means no final response arrived within the transaction timer. It can be generated by your own dialer (it never heard back from the trunk) or returned by the trunk (it never heard back from the next hop). Long post-dial delay (PDD) is the softer version of the same symptom: the call eventually rings, but late.

Common causes

• Firewall or NAT dropping SIP responses, so the dialer never sees 100 Trying or 180 Ringing.
• Wrong transport or port, or UDP fragmentation of large SIP messages.
• DNS resolution delays for the trunk hostname.
• Downstream route congestion or slow destinations, which shows up as long PDD on specific prefixes only.
• Excessive call attempts per second overwhelming either side.

How to diagnose

• If NO response at all arrives for any destination, treat it as connectivity: check firewall rules, source IP, host and port.
• If only certain prefixes are slow, record PDD per prefix and time of day and raise those prefixes specifically.
• Check whether 408s rise with load — if so, reduce CPS and re-test.

At CallCenterBoost

No PDD figure is published for any route. Measure PDD on your own test calls, and include prefixes, timestamps and observed delay in a VoIP Route ticket if it is consistently high.